The present invention relates to a display device, to an electronic apparatus using the display device, and to drive method.
Recently, liquid crystal displays, which are display devices of one of various kinds, have been widely used as light low-power-consumption display devices for electronic apparatuses such as television sets, electronic notebooks, personal computers and portable telephone sets. With respect to such liquid crystal displays, a further increase in the number of gray scale steps for improvement in the accuracy with which images are displayed is expected. Known techniques for realizing grayshade display with such liquid crystal displays are, for example, pulse height modulation of changing the height of pulses written to liquid crystal elements and pulse width modulation of changing the width of written pulses.
Recently, a drive method using a new system has attracted attention, which system is operated in a liquid crystal display using a nonlinear switching element such as a MIM element, a back-to-back diode element, a diode ring element or a varistor element in such a manner that, in a first mode, a first selecting voltage is supplied to a scanning line, and, in a second mode, a precharge voltage is supplied to the scanning line and a second selecting voltage is thereafter supplied to the scanning line. (This kind of method will hereinafter be referred to as a charge-discharge drive method.) For example, Japanese Patent Laid-Open No. 125225/1990 discloses a charge-discharge drive method. As described in this document, it has been considered that the mainstream of this kind of drive method is grayshade display by pulse height modulation. Pulse height modulation, however, entails the problem of difficulty in voltage control for display with a predetermined gray scale and the problem of a high cost of the liquid crystal display. On the other hand, a drive method which has been proposed or put to use before the charge-discharge drive method and which is called a four-value method because of use of a two-value selecting voltage and a two-value data voltage is also known. The idea of pulse width modulation in the four-value drive method, however, cannot be applied directly to the charge-discharge drive method.
The present invention has been achieved in consideration of the above-described problems, and an object of the present invention is to provide a display device having improved display characteristics, most suitable for the charge-discharge drive method and capable of grayshade display by pulse width modulation, an electronic apparatus using the display device, and a drive method.
To achieve the above-described object, according to the present invention, there is provided a display device including a plurality of scanning lines, a plurality of data lines, and display elements driven with the scanning lines and the data lines, the display device performing grayshade display by pulse width modulation, the display device comprising scanning signal drive means for supplying a first selecting voltage to the scanning lines in a first mode, and for supplying, in a second mode, a precharge voltage opposite in polarity to the first selecting voltage about a middle value of a data voltage applied to the data lines and thereafter supplying a second selecting voltage opposite in polarity to the precharge voltage about the middle value of the data voltage to the scanning lines, and data signal drive means for supplying a pulse-width-modulated data voltage to the data lines, wherein first and second write pulses formed by the first and second selecting voltages and data voltage in the first and second modes and setting the same gray scale value are such that, as the pulse width of one of the first and second write pulses is increased, the pulse width of the other is reduced and the rate of reduction of the pulse width of the other becomes lower.
The present invention enables driving of display elements using a so-called charge-discharge drive method. According to the present invention, as the pulse width of one of the first and second write pulses is increased, the pulse width of the other is reduced and the rate of this reduction becomes lower.
In this manner, the present invention enables suitable grayshade display using pulse width modulation while preventing application of a DC voltage to each display element over a long time period.
In the present invention, the precharge voltage may be positive or negative, and driving using a positive precharge voltage and driving using a negative precharge voltage may be mixedly performed.
The present invention also provides a display device including a plurality of scanning lines, a plurality of data lines, and display elements driven with the scanning lines and the data lines, the display device performing grayshade display by pulse width modulation, the display device comprising scanning signal drive means for supplying a first selecting voltage to the scanning lines in a first mode, and for supplying, in a second mode, a precharge voltage opposite in polarity to the first selecting voltage about a middle value of a data voltage applied to the data lines and thereafter supplying a second selecting voltage opposite in polarity to the precharge voltage about the middle value of the data voltage to the scanning lines, and data signal drive means for supplying a pulse-width-modulated data voltage to the data lines, wherein first and second write pulses formed by the first and second selecting voltages and data voltage in the first and second modes and setting the same gray scale value have pulse widths set to such values that voltages applied to each of the display elements immediately after the periods of selecting by the first and second selecting voltages are approximately equal to each other.
According to the present invention, the pulse widths of the first and second pulses are set to such values that voltages applied to each of the display elements immediately after the selecting periods (voltages applied at initial stages of holding periods) are approximately equal to each other with respect to the first mode and the second mode, thereby enabling suitable grayshade display using pulse width modulation.
The present invention also provides a display device including a plurality of scanning lines, a plurality of data lines, and display elements driven with the scanning lines and the data lines, the display device performing grayshade display by pulse width modulation, the display device comprising scanning signal drive means for supplying a first selecting voltage to the scanning lines in a first mode, and for supplying, in a second mode, a precharge voltage opposite in polarity to the first selecting voltage about a middle value of a data voltage applied to the data lines and thereafter supplying a second selecting voltage opposite in polarity to the precharge voltage about the middle value of the data voltage to the scanning lines, and data signal drive means for supplying a pulse-width-modulated data voltage to the data lines, wherein a DC component of the data voltage in one horizontal scanning period with respect to a middle voltage between an ON voltage and an OFF voltage is made approximately zero independent of the gray scale.
According to the present invention, the proportion of data signal formed of the ON voltage and the proportion of data signal portion formed of the OFF voltage in one horizontal scanning period can be made approximately equal to each other regardless of the display pattern, thereby effectively preventing occurrence of a vertical crosstalk or the like.
The present invention is also characterized in that the scanning signal drive means supplies, in the first mode, the first selecting voltage in a second period following a first period corresponding to the first half of one horizontal scanning period and equal in length to the first period, and supplies, in the second mode, the precharge voltage in a third period corresponding to the first half of one horizontal scanning period, and the second selecting voltage in a fourth period following the third period and equal in length to the third period, and that the data signal drive means keeps the data voltage at a low level with respect to the middle voltage of the ON and OFF voltages for a period in the first period equal in length to the period in the second period through which the data voltage is kept at a high level with respect to the middle voltage, keeps the data voltage at the high level for a period in the first period equal in length to the period in the second period through which the data voltage is kept at the low level, keeps the data voltage at the low level for a period in the third period equal in length to the period in the fourth period through which the data voltage is kept at the high level, and keeps the data voltage at the high level for a period in the third period equal in length to the period in the fourth period through which the data voltage is kept at the low level. In this manner, the DC component of the data voltage in one horizontal period can be made approximately zero independent of the gray scale, thereby preventing occurrence of a vertical crosstalk or the like. Advantageously, according to the present invention, the data signals in the first and third periods can easily be obtained by inverting the data signals in the second and fourth periods.
The present invention also provides an electronic apparatus comprising one of the above-described display devices. Thus, a display device used in an electronic apparatus such as a remote controller, an electronic calculator, a portable telephone set, a projector, or a personal computer can be improved in display characteristics and can be reduced in cost.